Individuals who suffer from a balance control deficit are abnormally prone to falling and have poor gait control when walking or engaging in other movement tasks. A balance control deficit may be the result of a wide variety of sensory and/or motor disorders that impair the posture and equilibrium control of the subject. In order to make a correct assessment of a subject's balance deficit, and thereby to take remedial measures, an examining physician, or physical therapist, must determine the subject's balance control ability for a number of motor tasks, such as standing, getting up out of a chair, walking down steps, etc. By observing the subject performing such motor tasks, the physician may be able to determine if the subject's balance control is within normal limits and, if not, how best to bring balance control near or within normal limits again. However, to provide a more accurate and objective assessment of the individual's sensory and motor components of posture and equilibrium, a test system which provides an objective quantifiable assessment of balance control is required.
Quantitative information on the human sense of balance can be obtained using a variety of methods and devices. Quantitative information on the efficacy of the human sense of balance can be obtained by the electrophysiological measurement of eye movements or of the postural responses of the limbs. A balance control deficit is indicated if a response is outside of the limits expected for individuals having a normal balance function. Quantitative postural information may also be obtained by measuring contractile activity of the muscles generating the internal body forces for maintaining the equilibrium position using electromyographic (EMG) recordings.
Balance deficits are, however, normally quantified by recording body sway, i.e., the displacement of the body from the equilibrium position. Quantification of the postural sway of a subject is known as "stabilometry" or "posturography". One such method for quantifying balance deficits involves the measurement of body sway in terms of displacement of the center of foot pressure (CFP), sometimes termed "center of force", generated by the inherent instability of a test subject standing on a fixed support surface. CFP is computed from the signals provided by force transducers which are typically embedded in the four corners of the support surface. The force transducer outputs are employed to obtain a projection, on the support surface platform, of the resultant forces acting at the subject's center of gravity. An anterior, front-to-back, projection is obtained by assuming that the difference between the force detected by the for and aft force transducer-pairs equals torque about the ankle joint. The anterior projection is obtained by dividing the ankle torque by the total vertical force. This calculation also assumes that the upright body can be represented by a simple upright pendulum. Thus, only the effect of movement at the ankle joints is considered, the effect of movements at the knee and hip joints is ignored. A similar calculation employs the signals provided by the lateral pairs of force transducers on each side of the support platform to obtain a lateral force projection. The vectorial sum of the anterior and lateral force projections equals the CFP. As body sway frequencies exceed 0.2 Hz, however, this method for estimating the movement of the body's center of gravity based on CFP becomes increasingly inaccurate, because oscillations of the upper body enter the CFP measurements as inertial reaction forces. Furthermore, if the multi-link nature of the body is ignored, serious errors in understanding a subject's balance disorders can occur.
Investigators have used different types of force platforms to analyze postural sway. Some such force platforms are specifically targeted towards tests for analyzing balance disorders caused by vestibular deficits. Quantitative examination of CFP data suggests that subjects having a unilateral vestibular balance deficit, e.g., a balance deficit caused solely by impairment of the vestibular end organs in the ear, perform within normal ranges when tests are employed using a fixed force sensitive support surface to perform the balance tests. For this reason, techniques have been introduced which make the control of spontaneous sway by a subject positioned on the CFP measuring support surface more difficult. These techniques make quantification of a vestibular balance deficit easier by interrupting the non-vestibular sensory inputs that the subject may otherwise use to maintain his balance. One such technique involves moving the support surface so that it is tilted (forward or backward) in relation to changes in the subject's CFP. This type of controlled platform instability may be obtained using a purely mechanical device, or with a more flexible electronic and computer controlled motor unit. The movement of the support surface platform disrupts the somatosensory inputs which would otherwise be available to the subject. A second technique involves the use of a movable visual surround, which surrounds the subject, and which is moved to follow the subject's body sway, as estimated by CFP measurement of the subject. This technique disrupts the visual stabilization inputs used by the subject to maintain balance control. By disrupting the somatosensory and visual inputs, a test procedure for analyzing a subject's balance control is able to focus more particularly on the vestibular balance control mechanism. Examples of such test systems and procedures are described in more detail in U.S. Pat. Nos. 4,738,269, 5,052,406, and 5,303,715 issued to Nashner, et al. Analysis of tests employing these improvements to CFP sway quantification have indicated that destabilization of the support surface beneath the subject provides a major diagnostic improvement. However, destabilizing a visual surround by moving it in relation to the CFP provides little additional diagnostic information as far as vestibular balance deficit is concerned.
A major drawback of CFP based systems for quantifying body sway is that the freedom of movement of the subject is limited by the fact that the subject must remain in contact with the force sensing support surface. Thus, the physician's ability to determine a subject's balance control while performing a variety of motor tasks is limited by the CFP method. A more flexible system that may be used to measure body sway employs light-weight light-emitting sources that are mounted on a subject's body. However, such three-dimensional camera based systems are typically prohibitively expensive for most physical therapy practices specializing in rehabilitation of gait and balance deficits. Moreover, these systems also have a number of technical drawbacks, including excessive computer power requirements, limited on-line capabilities, sensitivity to interfering light sources, and limited range of operation. Thus, although such systems are capable of quantifying gait and other dynamic postural abnormalities, which cannot be achieved using CFP measuring support surfaces, this advantage is outweighed by the price and ease-of-operation advantages of more conventional CFP systems. Thus, CFP systems are still the system of choice for quantifying the body sway of subjects with balance deficits.
After a balance deficit has been diagnosed and quantified, a physician may prescribe remedial measures to bring the subject's balance control near or within normal limits. The physician may prescribe medication that reduces the action of peripheral senses on the brain. Alternatively, the physician may prescribe a course of physical therapy, which will typically last at least several weeks, with the object of training the subject's brain to deal with a reduced sense of balance when trying to maintain the body upright and prevent a fall. However, neither of these techniques will have an immediate rehabilitory effect on the subject's balance deficit. Moreover, medication can have side effects, and can also reduce the capability of the brain to process balance information from the peripheral senses. A course of physical therapy requires a long training period which may extend over more than two months. These difficulties and limitations associated with conventional remedial measures for dealing with balance deficits are most problematic when the subject is older and likely to have a falling tendency.
In the field of hearing, which is physiologically related to that of balance, two types of prostheses are used to augment a subject's hearing ability. The first type of device involves augmenting the sound waves in the external ear canal so that they have greater excursions when they reach the inner ear where they are transduced into electrical signals that reach the brain. The second type of device for improving hearing ability involves direct electrical stimulation of the auditory nerve in the inner ear. For the sense of balance, however, in which sensory signals from different neurophysiological systems, including a major input from the vestibular system of the inner ear, are combined by the brain to yield a unitary sense of balance, no prosthetic device exists.